Broadband balanced frequency converters in the low radio-frequency (“RF”) to millimeter-wave frequency (“mm-wave”) range are vital to many types of electronic instrumentation. Typically, a frequency converter is a device that utilizes a mixer. A mixer is a multi-port non-linear circuit or device that accepts as its input two input signals having two different frequencies and in response produces at its output four signals that include an output signal having a frequency equal to the sum of the frequencies of the input signals, a second output signal having a frequency equal to the difference between the frequencies of the input signals, and, if they are not filtered out, the original input signals. In general, the output signals will have finite bandwidth around the input signal frequencies. As an example of operation, a mixer converts signals from one frequency to another by applying a frequency reference signal (generally known as local oscillator “LO” reference signal) to one port of the mixer, and the signal to be converted (generally known as a “base-band signal”) to a second port of the mixer. The sum and difference of the frequencies of the two signals are predominantly what appears at the output port of the mixer, although it is appreciated by those skilled in the art that the sum and difference of the frequencies of the harmonics of the two input signals and other higher-order products also co-exist at the mixer output port.
Generally for transmitter applications, a low frequency signal (i.e., the base-band signal) is first processed and then up-converted (i.e., modulated) through the mixer to an up-converted signal having a higher frequency (such as an intermediate frequency “IF” and/or microwave frequency), where the up-converted signal is usually amplified and transmitted. The modulation process includes combining the base-band signal with a high frequency carrier signal (that is generated from the frequency reference signal) to produce the up-converted signal (also known as a “modulated signal”).
For receiver applications, the received signal (which is a modulated signal) is down-converted through the mixer by removing the high frequency carrier signal (i.e., demodulated) to produce the low frequency base-band signal, where the information is extracted.
The dynamic range of a mixer is the input power range over which the mixer is usable. On the low-input power end, it is limited by the noise figure and other system parameters such as signal to-noise ratio (“SNR” or “S/N”) and receiver bandwidth. On the high-end it is limited by either the saturation level or the input level for which certain spurious signals reach unacceptable levels.
To cover a broad range of frequencies and to offer multiple transmitter/receiver channels, broadband frequency converters have utilized combinations of mixers to get adequate coverage. Unfortunately, this results in disadvantageous bulk and cost. Generally, modern instrumentation designs reflect a tendency toward remote instrument heads and higher levels of integration. To meet the design and performance requirements, there is a need for a broadband frequency converter having a mixer capable of both operating across broad RF and local oscillator frequency bandwidths and of being integrated with other like mixers into a single package.
Additionally, a balanced frequency converter is a device that utilizes a balanced mixer. Generally, electrical signals should be balanced to maximize power in many transmission situations. As such, balanced mixers have advantages over unbalanced mixers because balanced mixers generally have better power-handling capabilities, spurious rejection, noise rejection, and allow the LO and RF injection to be separated. It is appreciated by those skilled in the art that to achieve balanced LO and/or RF injection, generally a balun or hybrid is utilized to convert an unbalanced signal to a balanced one.
A balun is a device for converting an unbalanced signal to a balanced signal. When utilized in combination with a mixer, the degree of electrical balance produced by the balun is important in preventing input signal energy from exiting the wrong mixer ports. Ideally, the balun should not create large reflections or insertion losses. An example of a broad-bandwidth balun is described in U.S. Pat. No. 6,084,485, titled “Broad-bandwidth Balun with Polyiron Cones and a Conductive Rod in a Conductive Housing,” issued to Bickford et al. on Jul. 4, 2000, which is herein incorporated by reference in its entirety.
Therefore, there is a need for a broadband balanced frequency converter having a balanced mixer capable of both operating across broad RF and local oscillator frequency bandwidths and being integrated with other like balanced mixers into a single package.
Examples of known mixers are shown FIGS. 1 and 2. In FIG. 1, a schematic diagram of an example of an implementation of a known mixer is shown. In FIG. 2, a schematic diagram of an example of another implementation of a known mixer similar to the example mixer shown in FIG. 1 with a Wilkinson-type splitter is shown. Both of these example mixers are described in U.S. Pat. No. 6,205,324, titled “Broadband Double-balanced Frequency Mixer,” issued to J. Bickford on Mar. 20, 2001, which is herein incorporated by reference in its entirety.
Several approaches have been utilized in the past in an attempt to achieve the greatest possible bandwidth and performance while minimizing the cost and size of the designs. Generally, these approaches may be separated into either hybrid or monolithic approaches.
Hybrid approaches utilize modular components integrated into a single package and may include a mixer circuit 300 in combination with a passive balun circuit 302 as shown in FIG. 3. In FIG. 3, the balun module 302 is a passive device that balances the received frequency reference signal 304 and produces two frequency reference signals 306 that are 180 degrees out of phase. The two frequency reference signals 306 are then input into the mixer circuit 300 and mixed with a balanced input signal 308 that may be either a base-band, intermediate frequency (“IF”) signal, or radio-frequency (“RF”) signal. The mixer circuit 300 then produces output signal 310 that may be either a base-band, IF, or RF signal. It is appreciated by those skilled in the art that the mixer circuit 300 produces a base-band, IF, or RF signal is based on whether the mixer circuit 300 acts as a down-converter or up-converter. An example of operation of the mixer circuit 300 is described in sections 7.3 and 8.3 of “Microwave Mixers,” by Maas and published in 1993 by Artech House, which is herein incorporated by reference in its entirety. Unfortunately, these hybrid approaches have the disadvantage that the balun 302 size is too large, is difficult to integrate into a multiple-mixer circuit, or has bandwidth and signal limitations.
Monolithic approaches are integrated circuits (“ICs”) utilizing compatible process technologies such as, for example, a Gilbert Cell mixer or a distributed mixer. Unfortunately, these monolithic approaches have the disadvantage that monolithic mixers are generally limited by parasitic effects directly related to the device technology and, therefore, the device parasitic effects limit the bandwidth and performance of these monolithic approaches.
Therefore, there is a need for a frequency converter system that is balanced and achieves higher performance at a reduced size and cost than the previous approaches.